Sunday, December 9, 2012

Hidden In Plain Sight

Whilst I was more than happy with the performance of the scale speed trap I built back in August, I wasn't happy with how it looked. Whilst most of the electronics can be hidden out of the way the light dependent resistors (LDRs) have to be on the track somewhere. Whilst I had chosen fairly small and flat LDRs they were still quite large and unsightly. To have any chance of hiding them I would have needed to place them below the sleepers which would have permanently covered part of the working surface. So if I want to add the speed trap to a nicely modelled layout I have to find a way of fitting and hiding the LDRs.

Fortunately it turned out to be easy to find LDRs that were a better fit to the track; the VACTEC - VT935G are a push fit between the sleepers and cost me just 70p each (with a pull-down resistor included) from oomlout. This just left the small problem of how to disguise the LDRs on a layout.

When I catch the train to work I have to cross the tracks to reach the Sheffield bound platform. Right next to the crossing is a signal and, as with many signals, it is fitted with a Train Protection Warning System (TPWS). A full TPWS consists of four grids positioned at set distances from the signal. Firstly there is an Overspeed Sensor System (OSS) which uses two grids a short distance apart to determine the speed of a train approaching a signal. If the train is travelling two fast then the breaks are automatically applied. The second half of the system is a Train Stop System (TSS) which consists of two grids next to each other positioned at the signal which are triggered by any movement of the train. You can find full details of how TWPS works at this interesting web page.

While a TSS would probably place the two LDRs too close together, using two separate grids as an OSS to hide the LDRs seemed like a perfect way of hiding the electronics in plain sight. Now PECO do make TPWS grids in the right scale, but when I looked into these I felt that they were a little chunky for my liking. In reality, from anything other than close to, the grids actually look more like a set of parallel bars as the thin ties aren't really visible. The problem is that modelling such thin bars at 4mm to the foot scale would result in very very fine plastic bars -- too fine to model accurately and as a result the are oversize on the PECO grids.

Having had some success with 3D printing, I decided to have a go at creating my own TPWS grids. The simple structure was easy to model and has printed really well. While I chose to leave out the thin tie bars completely, I'm quite happy with the look of the resulting grids, and they are a perfect size to hide the LDRs from view. Once the track is properly ballasted and the grids are painted it should all come together nicely.

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